Unique user sessions are periods where unique website users spend a specific amount of time on the site. If they return within that specific time frame, it is still considered one session. It’s like a headcount of the number of people visiting your site in that particular period.
For example…
To celebrate the opening of his new online liquor shop, Zack decided to give away a bottle of wine to the first 100 visitors. Amongst other things he checked was the number of unique user sessions to ensure that he doesn’t give away bottles of wine to the same visitor twice.
